Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and a storage medium and, more particularly, to an image processing technique for medical images captured by various medical imaging apparatuses (modalities).
Description of the Related Art
Japanese Patent Laid-Open No. 2010-88627 describes a PAT (Photoacoustic Tomography Apparatus) (to also be referred to as a PAT apparatus hereinafter) as a kind of medical imaging apparatus. The PAT apparatus excites an absorbing substance in a subject by irradiating the subject with optical pulses, and detects a photoacoustic signal generated by the thermoelastic expansion of the absorbing substance, thereby imaging properties associated with the light absorption of the subject as a three-dimensional image (three-dimensional tomographic image). The PAT apparatus images the distribution of optical energy accumulation amounts (the distribution of optical energy absorption densities) in a subject with respect to irradiation light. In addition, based on this distribution, the PAT apparatus images the distribution of optical absorption coefficients of the subject concerning an irradiation wavelength. In addition, it is possible to image the states (for example, oxygen saturation of hemoglobin) of substances constituting the subject based on the distribution of optical absorption coefficients concerning a plurality of wavelengths.
These images are expected to visualize information associated with new blood vessels generated inside and outside a malignant tumor such as a cancer. These images will be collectively referred to as PAT images (photoacoustic tomographic images) hereinafter.
A PAT apparatus irradiates a human body with near-infrared pulses of low energy, and hence has difficulty in imaging a deep portion of the human body as compared with X-rays or the like. As a form of a PAT apparatus designed to measure breasts, the PAT apparatus described in Japanese Patent Laid-Open No. 2010-88627 images a breast while holding the breast by two flat plates (to be referred to as holding plates hereinafter) to reduce the thickness of the breast. Therefore, when performing a diagnosis using both the PAT apparatus and another medical imaging apparatus such as an MRI (Magnetic Resonance Imaging) apparatus, a doctor can efficiently perform the diagnosis by performing deformation alignment (deforming one image to conform to the other image) in consideration of compression deformation caused by holding the breast.
A method of aligning a PAT image with an MRI image includes an image matching method. For example, non-patent literature 1 (Angela Lee, et al., “Breast X-ray and MR image fusion using finite element modeling”, Proc. Workshop on Breast Image Analysis in conjunction with MICCAI 2011, pp. 129-136, 2011) describes a technique for alignment between an X-ray mammography (MMG) image obtained by imaging a breast compressed by flat plates and an MRI image of the breast. More specifically, a deformed MRI image is generated by performing, for an MRI image, a physical deformation simulation by compression with flat plates, a pseudo-MMG image is generated from the deformed MRI image, and alignment is performed by matching between the pseudo-MMG image and the actually captured MMG image.
In addition, non-patent literature 2 (C. Tanner, et al., “Breast Shapes on Real and Simulated Mammograms”, Proc. Int. Workshop on Digital Mammography 2010 (IWDM 2010), LNCS 6136, pp. 540-547, 2010) discloses a technique of evaluating, based on the two-dimensional shape of a breast which is extracted from an MMG image, the shape of the breast after deformation which is obtained as a result of performing, for an MRI image, a physical deformation simulation by compression with flat plates.
However, since an image obtained by a medical imaging apparatus (for example, a PAT apparatus) and an image obtained by another medical imaging apparatus (for example, an MRI apparatus) differ in their characteristics to be imaged, not all the structures depicted in the first image (MRI image) match those on the second image (PAT image). Therefore, even if the technique described in non-patent literature 1 is used for alignment between the second image (PAT image) and the first image (MRI image), it is difficult to execute accurate alignment by performing only image matching. On the other hand, since the second image (PAT image) does not include sufficient outer shape information of a subject, it is difficult to directly use the technique described in non-patent literature 2.
The present invention provides a technique of accurately aligning a first image (three-dimensional image (three-dimensional tomographic image)) such as an MRI image or CT image of a subject as an object with a second image (PAT image).